X-ray reflectometry (XRR) is a well-known technique for measuring the thickness, density and surface quality of thin film layers deposited on a substrate. Conventional X-ray reflectometers are sold by a number of companies, among them Technos (Osaka, Japan), Siemens (Munich, Germany) and Bede Scientific Instrument (Durham, UK). Such reflectometers typically operate by irradiating a sample with a beam of X-rays at grazing incidence, i.e., at a small angle relative to the surface of the sample, near the total external reflection angle of the sample material. Measurement of X-ray intensity reflected from the sample as a function of angle gives a pattern of interference fringes, which is analyzed to determine the properties of the film layers responsible for creating the fringe pattern. The X-ray intensity measurements are commonly made using a position-sensitive detector, such as a proportional counter or an array detector, typically a photodiode array or charge-coupled device (CCD).
A method for analyzing the X-ray data to determine film thickness is described, for example, in U.S. Pat. No. 5,740,226, to Komiya et al., whose disclosure is incorporated herein by reference. After measuring X-ray reflectance as a function of angle, an average reflectance curve is fitted to the fringe spectrum. The average curve is based on a formula that expresses attenuation, background and surface roughness of the film. The fitted average reflectance curve is then used in extracting the oscillatory component of the fringe spectrum. This component is Fourier transformed to find the film thickness.
U.S. Pat. No. 5,619,548, to Koppel, whose disclosure is incorporated herein by reference, describes an X-ray thickness gauge based on reflectometric measurement. A curved, reflective X-ray monochromator is used to focus X-rays onto the surface of a sample. A position-sensitive detector, such as a photodiode detector array, senses the X-rays reflected from the surface and produces an intensity signal as a function of reflection angle. The angle-dependent signal is analyzed to determine properties of the structure of a thin film layer on the sample, including thickness, density and surface roughness.
U.S. Pat. No. 5,923,720, to Barton et al., whose disclosure is incorporated herein by reference, also describes an X-ray spectrometer based on a curved crystal monochromator. The monochromator has the shape of a tapered logarithmic spiral, which is described as achieving a finer focal spot on a sample surface than prior art monochromators. X-rays reflected or diffracted from the sample surface are received by a position-sensitive detector.
XRR may also be used in situ, within a deposition furnace, to inspect thin film layers in production on a semiconductor wafer, as described, for example, by Hayashi et al., in U.S. Patent Application Publication US 2001/0043668 A1, whose disclosure is incorporated herein by reference. The furnace is provided with X-ray incidence and extraction windows in its side walls. The substrate upon which the thin film has been deposited is irradiated through the incidence window, and the X-rays reflected from the substrate are sensed through the X-ray extraction window.
Small-angle X-ray scattering (SAXRS) is another method for surface layer characterization. It is described, for example, by Parrill et al., in “GISAXS—Glancing Incidence Small Angle X-ray Scattering,” Journal de Physique IV 3 (December, 1993), pages 411-417, which is incorporated herein by reference. In this method, an incident X-ray beam is totally externally reflected from a surface. The evanescent wave within the surface region is scattered by microscopic structures within the region. Measurement of the scattered evanescent wave can provide information about these structures. For example, Parrill et al. describe the use of this technique for determining size information regarding islands associated with film growth on the surface.
SAXRS can be used in this manner to determine characteristics of pores in a surface layer of a low-k dielectric material formed on a silicon wafer. Nano-porous silicates and polymers are considered to be attractive materials for use in microelectronic devices with sub-0.25 μm technology, but non-destructive characterization of pore size and density has so far proved to be a difficult task. The use of diffuse X-ray reflectivity in characterizing porous low-k materials is described, for example, by Wormington in “Characterization of Pore Size Distribution in Low k Dielectrics Using X-ray Reflectivity,” presented at the Sematech Gate Stack Engineering Workshop (Austin, Tex., May 2, 2002), which is incorporated herein by reference. A similar method is described by Ito in “X-ray Scattering Method for Determining Pore-Size Distribution in Low-k Thin Films,” presented at the International Sematech Ultra Low-k Workshop (San Francisco, Calif., Jun. 6-7, 2002), which is also incorporated herein by reference.
The configuration of X-ray optics used to irradiate a sample under evaluation by SAXRS is typically different from that used in XRR. For example, Iwasaki describes an X-ray optical device and multilayer mirror for use in a small angle scattering system in U.S. Patent Application Publication US 2001/0028699 A1, now U.S. Pat. No. 6,504,902 B2, whose disclosure is incorporated herein by reference. The multilayer mirror has elliptical reflection faces, which have two focal points. Thus, an X-ray beam from a source at one of the focal points is focused to a spot at the other focal point in a manner that is said to provide high precision in small-angle scattering measurements.